Blender link external scene in the existing scene
We will use the link function (not the "append", which creates a full copy of your data and don't do any sync from the source file).


From your "master file", separate your assets into groups ;
Save each group alone in a separate "slave" file ;
In your master file, delete everything, then use the menu File → Link and link every group you need from your slaves files.


That way, your master file contains only linked groups. And your other slaves .blend files each contain one group, which you can edit. The master file will receive the changes. If you ever create a new object in a slave file, don't forget to assign it to the group so he will be synced.